Magnetic resonance imaging systems have become ubiquitous in the field of medical diagnostics. Over the last several decades the physics involved in magnetic resonance imaging has become well understood and increasingly sophisticated systems have been developed to produce high-quality useful images for medical purposes. Increasing work in the field concentrates on further improvement of the image quality and obtaining images which are acquired rapidly, with little patient discomfort, and which are even more useful for radiologists and physicians in identifying features within the patient's anatomy.
In general, MRI systems produce images by sensing emissions from gyromagnetic materials in the subject produced in response to radio frequency pulses in the presence of a primary magnetic field. The primary magnetic field is typically aligned with the patient's body and affects the precession of certain molecules in the patient's tissue. The alignment of these molecules with the magnetic field and their precession at characteristic frequencies dependent upon the field strength are the bases for the imaging physics. A series of gradient fields are produced by additional coils in the MRI system. These coils produce fields which vary in strength in predictable and controlled manners to produce field gradients. The field gradients are used to select a slice of interest to be imaged, and to encode the gyromagnetic material response as a function of frequency and phase. By processing the sensed emissions from the gyromagnetic material in response to radio frequency pulses, the influence on the gyromagnetic molecules encoded by the gradients permits the emissions to be analyzed to appropriately locate specific responses at specific positions in the slice. Through reconstruction techniques, then, a useful image can be produced which comprises an array of adjacent picture elements or pixels corresponding to volume elements or voxels within the selected slice. The reconstructed image may be saved in digital form, transmitted, printed, transferred to photographic film, and so forth, depending upon the desired end use.
Despite the advances in MRI systems, there remain difficulties in obtaining the desired image quality. For example, coordination of beginning and ending times of pulses generated during examination sequences is often difficult to control precisely. These pulses include both radio frequency pulses and pulses used to define the desired magnetic field gradients. While ideal pulse profiles and timing between pulses can be defined precisely, in actual implementation variations often occur in both the pulse profile and the pulse timing.
Such variations may have several causes. For example, timing coordination may be affected by the response of electronic circuitry used to drive the radio frequency and gradient coils. The circuitry typically includes analog-to-digital and digital-to-analog converters, digital and analog band limiting filters, amplifiers, and so forth. Another important source of pulse variations is residual or uncompensated eddy currents which may be produced by structures surrounding one or more of the coils of the MRI system. Such eddy currents result from changes in the magnetic fields generated by the coils, and will tend to be more pronounced when high amplitude and rapidly changing fields are generated. Not only are such eddy currents difficult to model, but they may vary between physical axes on a particular MRI system, as well as between axes on various systems, even of the same type of model.
Attempts have been made to compensate for relative timing delays or shifts, as well as for variations in waveforms resulting from such delays. For example, compensations for delays may be implemented through software used to define the pulse sequences of the MRI examinations. However, such solutions are not well suited to situations where the delays vary within and between particular systems. Rather, a single delay is commonly used for all systems, providing an approximation of the effects, but failing to account for system variations. As a result, image quality problems can occur when actual variations or delays differ from system to system or within a single system. For example, with certain RF excitation pulses, errors in relative timing between radio frequency pulses and gradient waveforms can cause intensity variations for important gyromagnetic materials, such as water, in off-center slices. Other image undesirable artifacts can also result from the errors in pulse profile and timing.
There is a need, therefore, for a technique for identifying and calibrating time delays between RF and gradient field pulses in MRI systems. There is a particular need for a technique which permits such time delays to be identified and calibrated in a fairly straightforward manner on a single axis or multiple axes of a single system, permitting customization of pulse sequences for individual systems and even individual axes.